Port qdoc's libclang detection to the configure system

This is just an initial rough port with a custom config detection. More
things can be refined in the future, for example turning libclang into
a library to be used with QMAKE_USE.

Fixes: QTBUG-72259
Change-Id: Iaf3c42f76b79bdfd07476173c2fd0f4060c11fea
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>